A risk factor is anything that increases someone's chance of developing a disease. Having a risk factor does not mean that a person will develop a disease, it simply means that they are at a higher risk of developing it. Some women with risk factors for breast cancer never develop the disease, while most women who develop breast cancer have no family history of the disease and no other special risks.

Women who have close relatives with breast cancer are at higher risk of developing breast cancer themselves. A close relative is considered a mother, daughter or sister. For example, a woman whose mother developed breast cancer will have approximately twice the normal lifetime risk of developing breast cancer. The more affected relatives a woman has, the greater her risk. The risk is also higher when the relatives are diagnosed with breast cancer at a younger age.

When breast cancer runs in a family, it may be due to genetic mutations that predispose her to the disease. Researchers have found that mutations in two genes — BRCA1 and BRCA2 — are associated with hereditary breast cancer. These genetic mutations occur in 2 percent of Ashkenazi Jewish women, whose ancestors are from Eastern or Central Europe. Blood tests can identify women who have the genetic mutations. Women with a strong family history of breast cancer who also have mutations in BRCA1 or BRCA2 have a 50 percent to 85 percent lifetime risk of breast cancer and an increased risk of developing ovarian cancer as well. A woman from one of these breast cancer-prone families who does not carry one of the genetic mutations has the same breast-cancer risk as women without a family history of the disease.

Women with strong family histories of breast or ovarian cancer should consider testing. If the mutations are present there are several options to consider to reduce your risk for developing breast cancer. There is study evidence that either surgical removal of both ovaries or of both breasts can significantly reduce your risk of developing breast cancer. Other women choose to take tamoxifen, a medication that blocks estrogen's effects. Another choice, that may be less effective than the others, is a combination of more frequent physical exam screenings with your doctor, mammograms and/or MRI. Since there are no definite answers as to what treatment works best, deciding what to do becomes as much a personal decision as a medical one. You should consult your health-care provider, an oncologist and a genetic counselor as well as your family when making a decision.

Aside from genetics, the risk of breast cancer increases in relation to exposure to estrogen, the hormone that governs a woman's reproductive life.

Estrogen levels rise as girls reach puberty and high levels of the hormone cycles in the bloodstream until menopause. The longer a woman is exposed to estrogen, the higher her risk of breast cancer. Thus, women who have late start of menstruation or periods, or early menopause have a decreased risk of breast cancer. Women who start menstruating at age 16 have about half the lifetime risk of breast cancer than those who started at age 12.

Women taking combination estrogen-progesterone hormone replacement therapy after menopause also have an elevated risk of breast cancer. Women taking oral contraceptive pills may also have a slightly increased risk of breast cancer. The small excessive risk from birth-control pills gradually resturns to zero risk 10 years after stopping use of this form of contraception.

Risks are higher in women who have not had children or were older than 30 during their first pregnancy. The reasons why early pregnancy seems to protect against breast cancer haven't been established, but one theory holds that the sooner the breast undergoes the hormonal changes associated with pregnancy, the less vulnerable it is to the effects of estrogen.

The heavier a woman is — especially after menopause — the greater her risk. Fat cells convert inactive compounds into estrogen. With increasing weight, there is more fat to produce more estrogen. Hormone levels rise and breast cancer risk rises.

Exposure to environmental pollutants could potentially increase breast-cancer risk, but no direct link has been found. In particular, scientists are researching the effects of chemicals used in pesticides (such as DDT), herbicides, plastics, paper, detergents, textiles, cosmetics, hair colorings and other common products, in the hopes of determining any possible relationship to breast cancer. There have been rumors that underarm antiperspirants and underwire bras can increase the risk of breast cancer, however, there is no evidence that this is true.

Even small amounts of alcohol daily increase a woman's risk of developing breast cancer. The risk is small for one drink per day. Women who drink two or more cans of beer, glasses of wine, or shots of hard liquor are 40 percent more likely to develop berast cancer, compared to women who don't drink any alcohol. the Nurses' Health Study suggested that higher ingestion of folic acid might offset some the increased risk related to alcohol.